Cretaceous and Early Cenozoic Stratigraphy and History of Eastern Kyŏngsang Basin, S. Korea

Abstract

A detailed stratigraphic work has made it possible to reconstruct the geology of the both sides of the Yangsan fault to the pre-fault state. The dextral motion of the fault of about 35 km is obvious since the geology fits very well if the eastern block is rearranged about 35 km northward. Such a restoration shows the original (i.e., pre-faulting) Yŏngyang subbasin characterized by the Yŏngyang-type stratigraphic scheme of the Hayang Group. This scheme is no more applicable beyond the southern boundary of the so-reconstructed Yŏngyang subbasin. The Eocene acidic tuff apparently was dissected by the Yangsan fault, which in turn was concealed by the mid-Miocene sedimentation. The northern peripheral part of the Yŏngyang subbasin is dislocated due to the Yangsan and the associated faults. But, a pre-fault reconstruction has figured out an east-west trending “Pyŏnghae trough” in which the Kyŏngjongdong and the Ullyonsan Formations deposited. It is presumed that the north-bounding scarp-making growth fault of the trough was forming but later turned into a reverse fault now called the ‘‘Ilwŏl thrust’’. A paleocurrent analysis with 158 cross-stratifications of the Kyŏngjŏngdong Formation and the overlying Hayang Group indicates a mean source area in the direction of N 54°E. Derived therefrom, reddish and variegated radiolarian chert pebbles abundantly occur in two separate intervals of the Hayang Group. The radiolarians secured from the chert pebbles range in age from Middle Permian to Early Jurassic. Coeval radiolarites, not at all found on Korean peninsula, are widely distributed in the Tamba-Mino-Ashio and Chichibu belts of Japan. Some of these belts may have been the source area of the study area.